fix: LCP source can be null

[kptdobe]

I can see null source in the data. Applying the same to target to be sure.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 36.90%. Comparing base (6552ddc) to head (8b9f125).

Files	Patch %	Lines
src/index.js	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##              1.x     #220   +/-   ##
=======================================
  Coverage   36.90%   36.90%           
=======================================
  Files           1        1           
  Lines         271      271           
=======================================
  Hits          100      100           
  Misses        171      171           

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